The purposes of this study were to assess and modify the original classification categories of the modified Oswestry Low Back Pain Disability Questionnaire (ODQ) and to determine the unidimensionality of the modified ODQ applying Rasch Analysis. The data were obtained from 108 work-related low back pain patients by physical therapists. Construct validity of the scale using the Rasch model required the structure of the rating scale to be modified from 6 response levels to 4 response levels. Eight items from the modified ODQ fit the Rasch model. The items, "pain intensity" and "social life" showed misfit statistics. In general, the order of item difficulty of the remaining 8 items showed a logical item difficulty hierarchy with the "changing degree of pain" item being the most difficult and the "walk" item being the easiest. The results showed that further study is needed to expand the construct of ODQ including additional higher-level items related to work activities. This study may be useful for establishing a standard method to assess the functionality of low back pain patients.